Sensitive and accurate methods for detecting molecular interactions are very desirable for a wide variety of applications, including drug discovery, environmental testing, diagnostics, gene expression analysis, genomics analysis, proteomics and for characterizing the binding of two molecules that are known to bind together. Several optical techniques for measuring molecular interactions at surfaces have been developed based on the evanescent field wave phenomenon.
One technique used is surface plasmon resonance, hereinafter referred to as SPR. The phenomenon of SPR is well known, and reviews may be found in, e.g. Homola, J., et al., Sensors and Actuators B 54: 3-15 (1999); Welford, K., Opt. Quant. Elect. 23:1 (1991); Raether, H., Physics of Thin Films 9: 145 (1977).
Typically, SPR is measured as a dip in intensity of light for a specific wavelength reflected at a specific angle from the interface between an optically transparent material, e.g., glass, and a thin metal film, usually silver or gold, and is dependent on the refractive index of the medium close to the metal surface. A change of the real part of the complex refractive index at the metal surface, such as by the adsorption or binding of material thereto, will cause a corresponding shift in the angle at which SPR occurs, the so-called SPR-angle. For a specific angle of incidence, the SPR is observed as a dip in intensity of light at a specific wavelength, a change in the real part of the refractive index causing a corresponding shift in the wavelength at which SPR occurs. In typical configurations, the medium close to the metal surface includes a sample which alters the refractive index of the medium close to the metal surface dependent upon the composition of the sample.
Three alternative arrangements may be used to couple the light to the interface such that SPR arises. These methods include using a metallized diffraction grating (see H. Raether in “Surface Polaritons”, Eds. Agranovich and Mills, North Holland Publ. Comp., Amsterdam, 1982), a metallized glass prism (Kretschmann configuration), or a prism in close contact with a metallized surface on a glass substrate (Otto configuration). In a SPR-based assay, for example, a ligand is bound to the metal surface, and the interaction of this sensing surface with an analyte in a solution in contact with the surface is monitored.
Other optical techniques similar to SPR are Brewster angle reflectometry (BAR) and critical angle reflectometry (CAR). When light is incident at the boundary between two different transparent dielectric media, from the higher to the lower refractive index medium, the internal reflectance varies with angle of incidence for both the s- and p-polarized components. The reflected s-polarized component increases with the angle of incidence, and the p-polarized component shows a minimum reflectance at a specific angle, the Brewster angle. The angle at which both s- and p-polarized light is totally internally reflected is defined as the critical angle. For all angles of incidence greater than the critical angle, total internal reflection (TIR) occurs.
Another optical technique similar to SPR is evanescent wave ellipsometry, described in Azzam, R. M. A., Surface Science 56: 126-133 (1976). In evanescent wave ellipsometry the intensity and polarization ellipse of the light reflected from the interface can be monitored as functions of the angle of incidence, wavelength or time. Under steady state conditions, measurements as a function of wavelength and angle of incidence can provide basic information on the molecular composition and organization of the medium close to the interface. In a dynamic time-varying situation, measurements as a function of time can resolve the kinetics of certain surface changes.
Evanescent wave sensors used in biochemical sensing applications typically have one or more ligands bound at or near the interface. The ligands are capable of selectively binding to the desired analytes. Binding of analytes by the ligands shifts the refractive index of the medium near the interface, thereby affecting the evanescent wave in a detectable fashion. Since the evanescent field wave penetrates only a short distance into the medium near the interface, the conditions for the evanescent wave sensors are relatively insensitive to changes in the bulk medium (distal from the interface). This provides a potential for very selective sensing of analytes based on selective ligand-analyte interactions.
Many methods are known for binding ligands at or near the interface. Representative methods are discussed in, e.g. Homola, J., et al., Sensors and Actuators B 54: 3-15 (1999); U.S. Pat. No. 5,242,828 to Bergstrom et al. (1993); and U.S. Pat. No. 6,738,141 to Thirstrup (2004).
Further literature of interest includes: U.S. Pat. No. 6,027,890 to Ness, et al. (2000); PCT publication WO97/27331; U.S. publication 20020117659; and the following papers: Olejnik et al., Proc. Natl. Acad. Sci. 92:7590-94; Olejnik et al., Meth. Enzymol. 291:135-154 (1998); Zhao et al., Anal. Chem. 74:4259-4268 (2002); Sanford et al., Chem. Mater. 10:1510-20 (1998); Guillier et al., Chem. Rev. 100:2091-2157 (2000); Fong et al., Analytica Chimica Acta 456:201-208 (2002); Ogata et al., Anal. Chem. 74:4702-4708 (2002); Bai et al., Nucl. Acids Res. 32:535-541 (2004); Cooper, Anal. Bioan. Chem 337:843-842 (2003); Homola, J., Anal. Bioan. Chem 337:528-539 (2003); Schultz, Curr. Opin. Biotechnol. 14:13-22 (2003); McDonnell, Curr. Opin. Chem. Biol. 5:572-577 (2001); Borch et al., Anal Chem 76:5243-5248 (2004); and Cui et al., Science 293:1289-1292 (2001).
A need still remains for further methods of providing sensors specific for desired analytes and methods of using such sensors.